A magnetic recording medium comprising a non-magnetic support having thereon a magnetic layer containing ferromagnetic particles dispersed in a binder is generally used as an audio tape, a video tape or a computer tape.
This type of magnetic recording medium is usually prepared by coating a magnetic coating composition containing a binder such as a resin and ferromagnetic particles dispersed in a solvent on a non-magnetic support to form a coated layer, providing the coated layer with a magnetic orientation treatment, a drying treatment and a surface smoothing treatment and thereafter slitting to a desired shape.
Recently, recording media with higher density have been developed to respond to the demand for increasing the recording density of recorded information. It is known that ferromagnetic particles of hexagonal crystalline ferrite are used in the magnetic recording medium to meet the above requirement, as disclosed in JP-B-60-50323. (The term "JP-B" as used herein means an "examined Japanese patent publication".) A magnetic recording medium using this hexagonal crystalline ferrite is very suitable as a perpendicularly magnetic recording medium. The hexagonal crystalline ferrite is a plate-like particle, and a magnetic recording medium using the above hexagonal crystalline ferrite suffers from more serious problems, as discussed below, than those exhibited by a magnetic recording medium prepared from conventional magnetic particles.
A tape of a high speed printing medium for DAT soft using hexagonal crystalline ferrite has been suggested as a high density recording medium. In this DAT soft (DAT tape), error rate is one of the important characteristics used to evaluate the efficiency of the tape. The error rate means a rate of misreading signals at the time of reproducing signals on the DAT tape. Seeing that the error rate increases as the number of drop outs occurring on the tape increases, it is known that there is a close correlation between the error rate and drop outs. Accordingly, it is important in a high density magnetic recording medium, such as the above described DAT tape, that the surface of the magnetic layer should be extremely smooth and the number of occurrences of drop outs should be extremely reduced.
It is generally considered that in a magnetic recording medium manufactured as described above, particle components are firmly fixed on the surface of the magnetic layer and the surface is extremely smooth. However, as a result of extensive studies by the present inventors, it was found that insufficiently fixed particle components, such as ferromagnetic particles, are present on the surface of the magnetic layer. These insufficiently fixed particle components happen to drop out during running of the tape and attach on a magnetic head, thereby causing magnetic head clogging, and drop outs in the case of a video tape. At this condition where drop outs occur, the error rate tends to increase in case of a DAT tape. Further, when ferromagnetic particles drop out, the amount of ferromagnetic particles present near the surface of the magnetic layer decreases, and there is also a problem that electromagnetic characteristics are decreased (decrease of output) after running repeatedly.
In this connection, the present inventors have found a method for grinding the surface of a magnetic layer as an approach to decreasing the drop out, head-clogging and output decrease and have already filed an application thereon (JP-A-62-172532). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.)
That is, the invention described in JP-A-62-172532 is to remove particle components which readily drop out and foreign matters attached on the surface of the magnetic layer to decrease the amount of substances which drop from the surface of the magnetic layer by grinding the smooth surface of the magnetic layer with a hard grinding tool such as a diamond wheel or a fixed sapphire blade.